Stuermer



May 07, 1957 K. sTUx-:RMER 2,791,023

LOWTENSION SPARK PLUG Filed may 1o, 1952 POM/[R Inventor As the first step in the conditioning process, the spark gap abutting the surface 38 of the insulator is provided with a transitory film or coating 54 of some suitable conductive material. This is shown in Figures A3 and 4. The transitory coating 54 may be graphite or carbon black or any other material which has similar electrical properties and which will be rapidly destroyed or removed by electrical discharges across the spark gap.

Any suitable means may be used to provide the coating 54. If carbon black or graphite is used, it can be brushed or smeared on to the insulatorsurface. Alternatively, the coating or lm can be put on in the form of an emulsion such as Aquadag (a water suspension of colloidal graphite) and the liquid medium subsequently evaporated. I have found it preferable to use this latter method since ya more uniform coating can thereby be attained.

By the term transitory coating as used herein is meant a coating such as will be destroyed or removed from the insulator surface during the subsequent conditioning steps to be hereinafter described. By the term conductive I intend also to include those materials which are often referred to as semiconductive.

Referring again to Figure l, the spark plug 4, after the coating 54 has been applied, is placed into an electrical circuit 5 as schematically shown and lan electrical discharge or a series of electrical discharges is applied across the spark gap. vThe amount of voltage necessary to initially cause a discharge across the spark gap will of course vary with the size of the spark gap along with the nature of the transistory coating 54. When graphite is used as the coating material the initial voltage necessary to cause discharge in the conditioning treatment will generally be somewhat less than the operating voltage for which th'e spark plug is designed after the conditioning treatment. Thus, for example, if the spark plug is designed for use at 2000 volts after conditioning, the initial voltage used in the conditioning treatment may be about 1500 volts.

During the application of the electrical discharge across the spark gap, the transitory coating 54 is Worn or burned off the surface of the insulator; however, by the time the destruction of the coating is completed, the spark plug will have been conditioned to operate at low voltage with the hard and wear-resistant lower portion of the insulator intact. t

The length of the conditioning treatment depends of course on the type of spark plug, on the amount of voltage used, and on the precise conditions under which the voltage is applied. For example,a short surge of anextremely high voltage might be used, or a somewhat longer surge of a lower voltage. Thus, the time can be anywhere from a fraction of a second up to a number of minutes.

It is preferable to use a series of electrical discharges for the conditioning treatment rather than one continuous discharge. I prefer to use repeated condenser discharges as the power source; however, either a capacitive or an inductive discharge system may of course be used. Polarity is not significant in the process; either the positive or negative pole may be connected to the center electrode.

It is often preferable during at least a portion of the conditioning process to submerge the tiring end of the spark plug in a liquid electrical insulating medium such as kerosene or other light insulating oil during the application of the electrical discharges. The insulating oil spark gap and this has been found to enhance the condi,- tioning treatment. Ordinarily, during operation in air, the discharge tends to bulge out of the gap due to magnetic and thermal etects. This is of course quite desirable during operation 'of the plug as an internal combustion engine igniter; however, during the conditioning treatment, it is preferable to confine the spark within the -serves the purpose of confining the discharge within the gap Iand this is what is accomplished hy the use of the liquid electrical insulating medium as above described.

The following example of a typical conditioning process will serve to better illustrate the invention:

Step 1,-Apply a coating of graphite to the spark gap abutting surface of the insulator by the application of Aquadag and subsequent evaporation of the water from the spark gap.

Step 2,-Spark the plug by the discharge of a con' denser of approximately 2 mfd. charged to a voltage of the order of 1500 volts at a rate of l0 to 30 sparks per second for about 30 seconds. Approximately 2 joules.

Step `iff-Spark the plug under oil (insulating oil or kerosene) by discharge of a condenser of approximately l mfd. charged to a voltage of the order of 1500 volts at a rate of l0 to 30 sparks per second for about one minute. Approximately 2 joules.

Step 4.-Spark the plug in air under identical condi tions as in Step 3 for about one minute or until the minimum sparking voltage of the plug is 500 volts or lower.

When the conditioning treatment is complete the original lm of conductive material will have been destroyed, the plug nevertheless continuing to operate as a low tension igniter. The spark plug conditioned by the above outlined process was of the type shown in Figure l and had a ygap size 'of .01 inch. After treatment, it operated eiciently as a creepage gap plug in a 2000 volt circuit and had an extremely long life.

It is to be understood that the above example is given merely for purposes of illustration and that other modifications can be used within the full and intended scope of the invention.

Exactly what occurs to the plug during the application of the voltage to destroy the transistory conductive lm and at the same time to render the plug adaptable for use in low tension systems is not fully known. It is believed that minute amounts of the electrode materials, or of the transitory film, or of the insulating oil decomposition products, or some combination of these, are spattered onto the surface of the insulator shown at 38 in Figure 2 -to render it semi-conductive. Whatever the precise nature of the spark plug 'or of the insulator surface might `be after the treatment is completed, it has been found that ythe thus conditioned plugs not only operate very efficiently in a low tension system but also have a greatly increased operating life over other and prior types of creepage gap plugs.

One reason for'the longer life of the spark plugs of this invention is that, in order to obtain creepage gap openation, no layer on the face of the insulator of the type usedr in prior low tension igniters is used. Because of this, the material for the spark gap abutting face of the insulator can be chosen solely for its properties of heat and wear resistance and mechanical strength. Thus, that part of the igniter most subject to wear and stress can be specifically designed to meet the high demand for strength and durability, and then by means of the novel process of this invention creep gap properties may be im- Various changes and modifications of the embodiments `'of the invention described herein may be made by those skilled in the art without departing from the spirit and principles of the invention.

I claim:

l. A process for treating a spark plug of the type compnsing a pair of electrodes having end portions in spaced relationship to form a spark gap therebetween and an electrical insulator separating said electrodes and having a surface portion interposed between said electrode end portions and adjacent said spark gap, said process cornprising the steps of providing said surface portion with a carbon coating, applying 'an electrical discharge across said spark gap while in air, submerging said spark gap in a liquid electrical insulating medium and applying an `eleirical discharge` across said spark` gap while submerged to condition said spark plug for operation lin a low voltage system.

2. A process for making a low tension spark plug of the Atype havin-g a pair of electrodes with end portions in spaced relationship to form a spark gap therebetween, comprising the steps of positioning a surface having good electrical insulating properties between said electrode end portions fand adjacent said spark gap, providing said surface with a graphite coating, applying a series of electrical discharges across said gap While in air, submerging said spark gap in a liquid electrical insulating medium and applying 'a series of electrical discharges across said gap while submerged and then removing said spark gap from said medium Iand applying a series of electrical discharges across said spark gap while in air.

3. A process for making a low tension spark plug of the type having ra pair of electrodes with end portions in spaced relationship to fonm a spark gap therebetween, comprising the steps of positioning a surface having good electrical insulating properties between said electrode end portions and adjacent said spark gap, applying a suspension of gnaphite to said surface and evaporating the liquid from said suspension to provide a film of graphite on said surface, applying a series of electrical discharges of about 1500 volts across said spark gap for about thirty seconds while in air, submerging said spark -gap in an electrical insulating oil and applying a series of discharges of about 1500 volts across said spark gap for about one minute while submerged and -then removing said spark gap from said medium and applying a series of discharges 'of about 1500 volts across said spark gap while in air.

4. A process for treating a spark plug of the type comprising a pair of electrodes having end portions in spaced relationship to form a spark gap therebetween and an electncal insulator separating said electrodes and having a surface portion interposed between said electrode end portions and adjacent said spark gap, said process comprising the steps of providing a transitory conductive coating on said surface portion, submerging the spark gap of said spark plug in a liquid electrical insulating medium and applying an electrical discharge across said spark gap while submerged to condition said spark plug for operation in a low voltage system.

5. A process tor treating a spark plug of the type comprising a pair of electrodes having end portions in spaced relationship to foam a spark gap therebetween and an electrical insulator separating said electrodes and having a surface porti'on interposed between said electrode end portions and adjacent said spark gap, said process comprising the steps of providing a transistory conductive coating 'on said surface portion, submerging the spark gap of said spark plug in a liquid electrical insulating medium and applying a series of electrical discharges across said spark gap while submerged to condition said spark plug for operation in a low voltage system.

References Cited in the le of this patent UNITED STATES PATENTS 1,537,903 Von Lepel May l2, 1925 2,168,206 Hastings Aug. 1, 1939 2,578,754 Smits Dec. 18, 1951 FOREIGN PATENTS 883,081 France June 23, 1943 

